1. Field of the Invention
The present invention relates to a liquid crystal composition (suitable for liquid crystal display elements), a process for production thereof, and a liquid crystal element (such as a liquid crystal display element) made therewith.
2. Description of the Related Art
Among liquid crystal materials used for liquid crystal display elements is a ferroelectric liquid crystal (FLC) which functions through the switching effect of the chiral smectic C phase (SmC* phase). It is attracting attention recently.
The FLC display is characterized by (1) quick response (1000 times as fast as the conventional nematic liquid crystal display), (2) less dependence on the viewing angle, and (3) image's memory function.
There is another liquid crystal attracting attention. It is a nematic liquid crystal which functions through the switching effect of the nematic phase (N phase). It is widely used for displays of various modes, such as twisted nematic mode (in-plane mode) with active matrix drive by thin film transistors (TFT), supertwisted nematic mode with simple matrix drive, guest-host mode with a dichroic dye, electrically-controlled birefringence (ECB) mode due to the effect of birefringence, dynamic scattering (DSM) mode, phase transition mode, and polymer dispersion mode.
There have hitherto been synthesized a variety of liquid crystal molecules which exhibit the ferroelectric properties. Such liquid crystals (FLC) were designed to reduce the response time (or increase the response speed). A conceivable means to achieve this object is to increase the spontaneous polarization (Ps) and decrease the viscosity in view of the fact that the response time (.tau.) of FLC is defined by .tau.=.eta./Ps.multidot.E, where .eta. is the viscosity of the material, Ps is the spontaneous polarization, and E is the electric field.
On the other hand, in the case of nematic liquid crystal, a conceivable means to reduce the response time (or increase the response speed) is to increase the dielectric constant and reduce the viscosity in view of the fact that the response time (.tau.) of nematic liquid crystal is defined by: EQU .tau..sub.on =.lambda..multidot.d.sup.2 /.epsilon..sub.0 .epsilon..sub.a (V.sup.2 -V.sub.c.sup.2) EQU .tau..sub.off =.lambda..multidot.d.sup.2 /.pi..sup.2 K
where .tau..sub.on is the rise time, .tau..sub.off is the breaking time, .lambda. is the viscosity, d is the cell gap, K is the elastic constant, and V.sub.c is the threshold voltage.
Liquid crystal display systems usually obey the above-mentioned equations as far as their response speed is concerned. However, with increased spontaneous polarization (Ps), they have difficulties with depolarization field that occurs in the liquid crystal display panel or disturbance of molecular orientation due to adsorption of impurity ions in the cell. In addition, increasing the dielectric constant poses problems with adsorption of impurity ions, sticking that occurs in the liquid crystal display panel, and disturbance of molecular orientation. The result is a speed lower than excepted or an insufficient contrast leading to a poor image quality. Because of this, it seems questionable to unduly increase the spontaneous polarization and dielectric constant.
One way now under study to decrease the viscosity of FLC and to increase the temperature range of FLC is to use achiral molecules (having no spontaneous polarization) and chiral molecules (having spontaneous polarization) in combination, with the former being as a base material and the latter being as a dopant, instead of using the former alone. (This is referred to as dopant method). In this case, however, the base material dominates in amount (concentration) over the dopant and hence affects the viscosity and temperature range.
Despite the foregoing, apparently it is chiral molecules having spontaneous polarization which governs the response of FLC to the electric field and determines the characteristic properties of FLC displays.
The above-mentioned equation has been used as a guide to selecting an adequate material from the standpoint of response speed; however, some consider it questionable after the discovery of many systems not conforming to it. In addition, there is no guide to selecting an adequate material from the standpoint of display contrast.
Another disadvantage of the FLC element is great dependence on temperature for switching. (This dependence is one order of magnitude greater than that of twisted nematic liquid crystal). This disadvantage must be overcome before the FLC element is put to practical use.
The viscosity of liquid crystal decreases with increasing temperature, resulting in the latching pulse width (or the pulse width to produce the memory effect) decreasing and the threshold voltage also decreasing simultaneously. (The threshold voltage depends on the latching pulse width.) Conversely, the viscosity of liquid crystal increases with decreasing temperature, resulting in the latching pulse width increasing and the threshold voltage increasing. It follows therefore that the temperature dependence of threshold voltage can be best avoided by eliminating the temperature dependence of viscosity. This is an effective way of approaching from the standpoint of material.
A conventional way to decrease viscosity was by introduction of a fluorine atom to the asymmetric carbon atom of the chiral molecule (as reported by Nohira et al. of Saitamo University). The modification of molecular structure, however, will make it difficult to meet a variety of requirements for display (such as contrast, response speed, bias stability, temperature range, temperature characteristics, and liquid crystal orientation).
In the case of nematic crystal liquid, a common way to decrease viscosity is by blending rather than the modification of molecular structure. However, blending still has difficulties in meeting the above-mentioned requirements.
A conceivable efficient way of material development is to pick up a material having good properties except for temperature characteristics and response speed and to make improvement on response speed.